I’m one of those lucky people who occasionally get to fly in their dreams. Such dreams are infrequent, they are vivid and thrilling, and I am inevitably sad to wake up. My dream flights – – whether they are happening inside an airplane or merely my own skin – – are always effortless and utterly carefree. If I’m flying without the help of machinery I may move my arms in a flapping motion or not; the slightest adjustment in my body may affect the direction my body is moving. I can go higher just by thinking about it, or I may have to push off with my toes, look upward, or raise my shoulders, ever so slightly. On the other hand, when I dream of flying airplanes there is always an element of uncertainty, and an edge of fear that something could go wrong.
Birds seem to have no such concerns, although looks can be deceiving. I once watched a seagull get caught momentarily by a twirling air current as it tumbled in a somersault over a rooftop. It righted itself and was flying fully in control again practically before I realized what I had seen. In their pursuit of life in the sky birds have evolved some of the most highly specialized adaptations found in the animal kingdom. Virtually no aspect of their anatomy or physiology is untouched by the modifications that allow for flight.
The most fundamental “unit” of flight is that structure which distinguishes birds from all other creatures: the feather. With the calamus providing a secure anchor into the skin, a flight feather’s rachis, or central shaft, extends distally from the body. The vanes to each side of the rachis are made up of barbs, each one a network of interlacing rows upon rows of barbules with tiny barbicels, or hooks, that latch onto neighboring barbule extensions (see Frolicking with Feathers – Part 1), functioning much like Velcro in keeping the feather’s structure intact through preening and the disturbances of flight.
Feathers are composed of keratin which makes them lightweight and firm, yet flexible. Various types of feathers are found on different parts of the body – some, like the primary flight feathers, formed with an asymmetry that maximizes the wing’s airfoil performance. Other kinds of feathers serve to insulate the body (downy feathers) and to further refine the wing’s and body’s shape and efficiency (contour feathers). Another feather type, the filoplumes, are paired with contour and flight feathers to monitor position and movement.
The wing loading, or relationship between wing area and body mass, is not isometric in birds. Birds of many different sizes, mass and wing shape are able to fly, and the changes in ratio among them somewhat defy the laws of physics. Since the wing area increases as the square of two dimensions, while the body mass increases with the cube of three dimensions, it could be predicted that in order to maintain parity with mass, the wing area must increase 1.5 times for every unit increase in mass. But this relationship holds true only in hummingbirds, whose wing area does, in fact, increase in proportion to mass. In most other birds the wing area may increase as little as 1.2 times for every unit increase in mass.
The result of these relationships is that birds as different as buzzards and swiftlets each exhibit their own unique distribution of surface area and rations of wing length to mass, and all of them can fly, albeit with very different styles of movement.
Birds such as storks, eagles and vultures have a high degree of slotting in their wings, and their long secondary wing feathers provide a significant amount of lift.
Hawks, vultures and ospreys all have the heavier body type that requires long secondary and highly slotted flight feathers in order to remain aloft over long distances.
Swifts, falcons, ducks and terns have distinctly pointed wings, their shape maximizing lift and minimizing drag to enable sustained flight at high speeds.
Sharply pointed wings give speed to nighthawks (Chordeiles minor) for catching insects on the wing, and to marbled godwits (Limosa fedoa) for migratory flights up to two thousand miles long.
The majority of species have an elliptical shaped wing to give high maneuverability in short bursts of activity.
Pine siskins (Spinus pinus) and a Cliff swallow (Petrochelidon pyrrhonota) show the typical elliptical wing shape that allows great flexibility of movement.
Many sea birds, such as gannets, gulls and albatross, use the high aspect ratio (ratio of length to width) of their wings to maintain lift for hours – – or days – – at a time while soaring over the oceans.
The wandering albatross, in fact, has a wingspan of 12 feet, the largest of any living bird species. These wings enable the wandering albatross to soar 500 miles in one day while maintaining speeds of almost 80 mph for eight straight hours without even a single wing flap.
(For more fun facts about albatross, see https://www.treehugger.com/albatross-facts-5073421.)
Many questions remain to be answered about the design, mechanics and energetics of bird flight. Maybe there will be a time when we have a fuller grasp of how birds do what they do so masterfully. The gyroscopes we use today will seem primitive, and we will have machines that can duplicate the hummingbird’s ability to both hover and maneuver with the most miniscule adjustments. Maybe there will be a time when all of us can do more than merely dream of flying, utterly effortless and carefree.
Bird photos courtesy of Mark Fuller